This invention relates to coding methods and, more particularly, to a dc-tree coding method which also provides channel gain.
Spread spectrum radio systems employ a coding technique wherein a transmitter modulates data to be transmitted over a radio frequency (RF) link with a spreading code. In order to recover the data, the receiver must inverse-spread the received signal by using the same spreading code as used by the transmitter. Spreading codes comprise strings of bits known as xe2x80x9cchipsxe2x80x9d which are multiplied with the data bits prior to transmission. Various spreading codes are known. Ideally, a spreading code should be balanced; that is, the occurrence of a positive chip xe2x80x9c+1xe2x80x9d is substantially equal in magnitude to that of a negative chip xe2x80x9cxe2x88x921xe2x80x9d. If the spreading code is not balanced, the received signal will include a direct current (DC) offset component having an amplitude corresponding to the imbalance.
A recently developed spread spectrum radio system minimizes RF component costs but requires dc-free baseband signaling with frequent polarity changes. This problem is similar to that found in previous magnetic recording and certain telecommunications systems and is solved with a method known as line coding.
The modulator for such spread spectrum radio system must generate two carriers with a frequency difference determined by channel parameters. One carrier is modulated with an FSK signal (frequency shift key) and spread while the other is just spread. The absolute accuracy of the carrier frequencies is not critical to system operation but the frequency difference is.
One low cost implementation is to use two frequency synthesizers and phase-locked-loops driven by a low stability reference oscillator. The FSK modulation is applied to one of the phase-locked loops at the voltage controlled oscillator (VCO) . Frequency components of this modulation that are within the loop filter bandwidth become part of the VCO control voltage and are nulled out. Consecutive strings of ones or zeros in the data field appear to droop when observed at the demodulator. Since both phased-locked loops share the reference oscillator, applying the low frequency components of the modulation to the reference oscillator signal as is done in classical synthesizer design requiring dc response is not applicable.
It has been demonstrated in the lab that Manchester coding circumvents the problem by restricting sign changes to no longer than one bit period. A disadvantage of Manchester coding is that the bandwidth required to transmit the baseband signal is doubled. This is acceptable since low data rates are expected and spectrum spreading is necessary for compliance with the U.S. Federal Communications Commission (FCC) rules. Nevertheless, if one is prepared to double the bandwidth, there may be another method which has the advantages of Manchester coding but also provides coding gain that will allow the system to operate with less transmit power, or, alternatively, allow the radio system to communicate over a greater range.
Methods have recently been published which offer dc free signaling and coding gain. Unfortunately, these were designed for higher order modulations or complicated code structures which are not applicable to the low cost transceiver of interest here.
It would therefore be advantageous to provide a dc-free coding scheme having channel gain.
In a preferred embodiment the invention, all N-dimensional vectors based on an alphabet of two symbols, where N is even, and with a maximum run of two symbols, are derived. That is, a list of all of the N-dimensional vectors where no more than two like symbols ever appear consecutively is derived. From this list, all vectors that begin or end in two identical symbols are eliminated. This avoids runs of longer than two consecutive symbols when vectors are concatenated. With this scheme, if the baseband transmission of a xe2x80x9c+xe2x80x9d symbol is realized with positive voltage V and transmission of a xe2x80x9cxe2x88x92xe2x80x9d symbol is realized with a negative voltage xe2x88x92V, then any concatenation of symbols is dc-free. Improved coding gain is realized by mapping modulated signals onto convolutional code trellises.